Thermal interfaces in microelectronics packages are commonly credited with a majority of the resistance for heat to escape from the chip to an attached cooling device (e.g. heat sinks, spreaders and the like). Thus, in order to minimize the thermal resistance between the heat source and cooling device, a thermally conductive paste, thermal grease or adhesive is commonly used. Thermal interfaces are typically formed by pressing the heat sink or chip cap onto the backside of the processor chip with a particle filled viscous medium between, which is forced to flow into cavities or non-uniformities between the surfaces.
Thermal interface materials are typically composed of an organic matrix highly loaded with a thermally conductive filler. Thermal conductivity is driven primarily by the nature of the filler, which is randomly and homogeneously distributed throughout the organic matrix. Commonly used fillers exhibit isotropic thermal conductivity and thermal interface materials utilizing these fillers must be highly loaded to achieve the desired thermal conductivity. Unfortunately, these loading levels degrade the properties of the base matrix material (such as flow, cohesion, interfacial adhesion, etc.).
It has been determined that stacking layers of electronic circuitry (i.e. 3 dimensional chip stack) and vertically interconnecting the layers provides a significant increase in circuit density per unit area. However, one significant problem of the three dimensional chip stack is heat dissipation from the inner chips. For a four layer 3 dimensional chip stack, the surface area presented to the heat sink by the chip stack has only ¼ of the surface area presented by the two-dimensional approach. For a 4-layer chip stack, there are three layer-layer thermal interfaces in addition to the final layer to grease/heat sink interface. The heat from the bottom layers must be conducted up thru the higher layers to get to the grease/heat sink interface.
One approach utilizes nanotubes, such as for example carbon nanotubes (CNTs), to promote heat dissipation from the inner chips. However, the CNTs are randomly oriented in the thermal interface material (TIM). CNTs and other thermally conductive carbon structures exhibit anisotropic thermal conductivity such that the thermal conductivity is orders of magnitude greater along one axis. Random distribution of the CNTs does not maximize the thermal conductivity of the TIM. Recently, another approach has been disclosed for the alignment of carbon structures which allows for 3D chip stacks to have aligned CNTs or any form of carbon nanofibers (CNF), such as for example graphite nanofibers (GNFs), in the xy plane, such that heat may be brought to the edges of the stack. However, the viscosity of the TIM makes alignment difficult.